If it's about writing itself, I typically get a 3k word chapter done in 3-6 hours. Usually spread across two or three days, but that's only usually. I can do 2 chapters a day or one chapter per 2 weeks sometimes.
After that though comes a lot of edits, most of them as I write the next few chapters and realize I haven't added something obvious that should have been there. Those can take up to months, sometimes even after the chapter in question has been published.
It's almost like everyone's having ADHD
ADHD is a spectrum, not a singular state. Depending how you narrow it down, you may even say that half of human population has varying symptoms at various levels of severity.
I, for one, have seen many signs of what can be called ADHD in myself, but I was never diagnosed for it.